ripngd: fix interface wakeup after shutdown

ripngd: fix interface wakeup after shutdown

RIPNG schedules a call to `ripng_interface_wakeup` in 1 second after
receiving the interface UP event from zebra. The function is called even
if the interface was shut down during this interval.

This is incorrect and also leads to a crash in the following scenario:
```
vtysh -c "conf" -c "router ripng vrf red" -c "network enp2s0"
ip link add red type vrf table 1
ip link set enp2s0 vrf red
ip link set enp2s0 down
ip link set enp2s0 up && sleep 2 && ip link del red
```

Signed-off-by: Igor Ryzhov <iryzhov@nfware.com>